Integrated circuits ("IC's") and discrete semiconductor devices such as transistors are fabricated on substrate wafers. Using techniques well known in the art, raw silicon is refined and grown into a single-crystal cylinder (known in the art as a "boule") whose diameter may range from about 4" to 12" or more. The cylinder is then sliced into a plurality of single-crystal wafers that are lapped, chemically etched and polished to form finished wafers.
The finished wafers serve as the starting substrate upon which various semiconductor devices and integrated circuits may be formed. During their fabrication and subsequent handling, the wafers are loaded into a cassette that typically can hold up to 25 wafers. FIGS. 1A and 1B depict a cassette 2 respectively in vertical and horizontal orientations loaded with wafers 4.
As used herein when the wafers are vertically oriented as shown in FIG. 1A, the cassette is said to be vertically oriented, and when the wafers are horizontally oriented as shown in FIG. 1B, the cassette is said to be horizontally oriented. An "arrow marker" 6 is shown in FIG. 1B to ease following the change in orientation and rotation of the cassette. It will be recognized, though, that actual cassettes probably will not include such an arrow marker.
Cassette 2 in FIGS. 1A and 1B is a representation of an actual cassette, although most cassettes can hold up to 25 finished wafers. For ease of illustration, cassette 2 is depicted as having the capacity to hold 9 rather than 25 wafers. Only six finished wafers are shown to illustrate that it is not necessary that all of the wafer positions be used.
Formation of semiconductor devices and ICs upon the finished wafers occurs at a semiconductor fabrication facility, referred to in the art as a "fab". A modern fab can include 150 work stations, at which the wafers undergo specific processes, e.g., introduction of dopant materials, formation of oxides, etching, wafer sorting, and so forth. FIG. 2 depicts a generic fab 8 as including a number of workstations WS1, WS2, WS3. Associated with each workstation is at least one robotic arm mechanism 12, often referred to as an "end effector" or "extractor". In general, mechanism 12 unloads wafers from a cassette and reloads the wafers into the cassette after processing at the workstation.
Wafers to be treated are typically transported between workstations in a cassette. To protect the wafers against spillage, the wafer-loaded cassettes are moved between workstations in a vertical orientation as shown in FIG. 1A. This inter-workstation orientation is depicted in FIG. 2 by vertically-oriented cassettes 2-V, which hold wafers 4-V, the "V" notation indicating the vertical orientation of the cassette. In the present state of the art, the vertically oriented wafer-loaded cassettes typically are hand-carried between workstations by human operators.
The workstations have a generally horizontal work surface 10 upon which one or more wafer-loaded cassettes 4 are placed in a horizontal orientation, such as shown in FIG. 1B. Robotic arm 12 then withdraws a horizontally oriented wafer, e.g., wafer 4-H, from a generally horizontally oriented cassette, e.g., cassette 2-H, the "H" notation indicating the horizontal orientation.
The robotic arm then moves the wafer into a working position at the workstation for processing. Workstation WS1, for example, is depicted as including mechanism denoted as WS1-P1 that performs a specific procedure upon each horizontally oriented wafer 4-H in a cassette 2-H. Workstation WS2 is shown as generically including two mechanisms, WS2-P1 and WS2-P2, that each perform a process upon the horizontally oriented wafers at that station.
Thus, at each workstation the cassette must be tilted from a vertical orientation required for safe wafer handling between workstations, to a generally horizontal orientation required by a workstation robotic arm. Generally, the required tilt angle will be about 90.degree.. In addition, once horizontally oriented, it will usually be necessary to rotate the wafer-loaded cassette through some angle .theta. about the vertical Z-axis for proper presentation of the wafers to the robotic arm. The rotation angle .theta. will generally range from about 0.degree. to 90.degree.. Mechanical guides 14 that are attached to the horizontal workstation surface 10 can assist the operator in achieving the necessary degree of rotation about the vertical axis.
In a present-day fab, a small percentage (perhaps 10%) of the workstations may include a mechanism that receives the cassette in a vertical orientation and tilts the holder 90.degree. into a horizontal orientation. However, the majority of workstations require that the human operator carrying the cassette, manually tilt the cassette approximately 90.degree. (to achieve a horizontal orientation) and also rotate the cassette some angle .theta. about the vertical axis to achieve a proper orientation for the robotic arm to access the now horizontally oriented wafers.
Unfortunately, manually tilting and rotating a wafer-loaded cassette can be ergonometrically challenging. Fab procedures dictate not only minimum clean room standards, but also minimum ergonometric standards for the human operators who hand-carry cassettes. A cassette fully loaded with wafers can present a sufficiently heavy load that precludes tilting 90.degree. and rotating through an angle .theta., in a sound ergometric manner. For example, a cassette containing 25 12" diameter wafers can weigh perhaps 20 pounds (9 Kg). In the future, when even larger diameter cylinders can be grown, manually tilting and rotating a fully loaded cassette may not be ergonometrically feasible. In addition, at some workstations, the space in which the wafer-loaded cassette must be tilted and rotated may be quite limited. Such space limitations can further hamper operator maneuverability in tilting and rotating a loaded cassette.
In future fabs, it is likely that an automated conveyer belt will replace human operators in transporting the cassettes, in a vertical orientation, from workstation to workstation. However, upon arrival at the majority of the workstations, the wafer-loaded cassettes will still have to be tilted 90.degree. into a horizontal orientation and then be rotated some angle .theta. about the vertical axis. Upon completion of processing at a given workstation, the cassette must then be tilted 90.degree. from a horizontal to a vertical orientation, counter-rotated through an angle -.theta. about the vertical axis, and then returned to the conveyor belt.
In summary, there is a need for a mechanism that can tilt a wafer-loaded cassette approximately 90.degree. from a generally vertical to generally horizontal orientation. Such mechanism should simultaneously rotate the cassette through a desired angle .theta. about the vertical axis for proper presentation of the wafers to the robotic arm of a workstation. After workstation processing is complete, the mechanism should receive the horizontally oriented cassette, and then provide a counter-tilt and counter-rotation of -.theta. about the vertical axis, and counter-tilt the holder approximately 90.degree., returning the cassette to a vertical orientation. Preferably such mechanism should not require excessive workstation space for implementation and operation.
The present invention discloses such a mechanism.